Switching power supplies (SPS) have been widely used in electronic products like information products and electric appliances. Due to the requirements for energy conservation and compactness of electronic products, how to enhance the working efficiency and power density of the SPS becomes the most important goal in this industry.
For an existent DC power supply like an AC to DC switching power supply, in order to shrink the size of a transformer, high-frequency pulse width modulation (PWM) is exploited to control the DC output voltage. FIG. 1 shows a conventional forward power converter circuit comprising a power switch Q1, a transformer T1, diodes D1, D2 and D3, an output inductor L and an output capacitor C. Its working principle is described as follows. When a PWM controller U1 turns the power switch Q1 on, an input voltage V1 provides power to a primary side winding coil N1 of the transformer T1. A current gradually builds on the winding coil to store energy therein. At the same time, a voltage having the same polarity with the primary side winding coil N1 will be induced at a secondary winding coil N2 of the transformer T1. The energy is thus forward-transferred to the secondary side winding coil N2, through the diode D1 and the output inductor L, and then to the load. At this time, the diode D2 is reverse biased, while the diode D1 is forward biased.
When the power switch Q1 is off, the polarity of the winding coil on the transformer T1 will reverse to let the diode D1 be reverse biased and thus off. The diode D2 will be on. The diode D2 is called a flywheel diode. At this time, the energy to the load is provided by the energy stored in the output inductor L and the output capacitor C via the diode D2. The output inductor L is an energy storage component.
Reference is again made to FIG. 1. The function of the transformer T1 is to isolate the primary side circuit and the secondary side circuit. Moreover, the required output voltage of the load can be obtained through the turn ratio of winding coils.
Reference is made to both FIGS. 1 and 2. When the diode D1 is on, a reverse conduction voltage Vrrm can be measured at two ends of the diode D1, and a conduction current Ic can be simultaneously measured. As shown in FIG. 2, the time from t0 to t1 is the switching time for conduction of the diodes D1 and D2. At this time, the conduction current Ic of the diode D1 will rise gradually, and the reverse conduction voltage Vrrm remains at the high level. Therefore, the converter will have a large switching loss during the switching period of the diodes D1 and D2.